The skin of Amphioxus

Three-dimensional fine structure of fibroblasts and other mesodermally derived tissues in the dermis of adults of the Bahamas lancelet (Chordata, Cephalohordata), as seen by serial block-face scanning electron microscopy

Tissues of adult cephalochordates include sparsely distributed fibroblasts. Previous work on these cells has left unsettled such questions as their developmental origin, range of functions, and even their overall shape. Here, we describe fibroblasts of a cephalochordate, the Bahamas lancelet, Asymmetron lucayanum, by serial block-face scanning electron microscopy to demonstrate their three-dimensional (3D) distribution and fine structure in a 0.56-mm length of the tail. The technique reveals in detail their position, abundance, and morphology. In the region studied, we found only 20 fibroblasts, well separated from one another. Each was strikingly stellate with long cytoplasmic processes rather similar to those of a vertebrate telocyte, a possibly fortuitous resemblance that is considered in the discussion section. In the cephalochordate dermis, the fibroblasts were never linked with one another, although they occasionally formed close associations of unknown significance with other cell types. The fibroblasts, in spite of their name, showed no signs of directly synthesizing fibrillar collagen. Instead, they appeared to be involved in the production of nonfibrous components of the extracellular matrix-both by the release of coarsely granular dense material and by secretion of more finely granular material by the local breakdown of their cytoplasmic processes. For context, the 3D structures of two other mesoderm-derived tissues (the midline mesoderm and the posteriormost somite) are also described for the region studied.

Evolution of the angiopoietin-like gene family in teleosts and their role in skin regeneration

BACKGROUND

The skin in vertebrates is a protective barrier and damage is rapidly repaired to re-establish barrier function and maintain internal homeostasis. The angiopoietin-like (ANGPTL) proteins are a family of eight secreted glycoproteins with an important role in skin repair and angiogenesis in humans. In other vertebrates their existence and role in skin remains largely unstudied. The present study characterizes for the first time the homologues of human ANGPTLs in fish and identifies the candidates that share a conserved role in skin repair using a regenerating teleost skin model over a 4-day healing period.

RESULTS

Homologues of human ANGPTL1-7 were identified in fish, although ANGPTL8 was absent and a totally new family member designated angptl9 was identified in fish and other non-mammalian vertebrates. In the teleost fishes a gene family expansion occurred but all the deduced Angptl proteins retained conserved sequence and structure motifs with the human homologues. In sea bream skin angptl1b, angptl2b, angptl4a, angptl4b and angptl7 transcripts were successfully amplified and they were differentially expressed during skin regeneration. In the first 2 days of skin regeneration, re-establishment of the physical barrier and an increase in the number of blood vessels was observed. During the initial stages of skin regeneration angptl1b and angptl2b transcripts were significantly more abundant (p < 0.05) than in intact skin and angptl7 transcripts were down-regulated (p < 0.05) throughout the 4-days of skin regeneration that was studied. No difference in angptl4a and angptl4b transcript abundance was detected during regeneration or between regenerating and intact skin.

CONCLUSIONS

The angptl gene family has expanded in teleost genomes. In sea bream, changes in the expression of angptl1b, angptl2b and angptl7 were correlated with the main phases of skin regeneration, indicating the involvement of ANGPTL family members in skin regeneration has been conserved in the vertebrates. Exploration of the fish angptl family in skin sheds new light on the understanding of the molecular basis of skin regeneration an issue of importance for disease control in aquaculture.

Development of somites and their derivatives in amphioxus, and implications for the evolution of vertebrate somites

Background

Vertebrate somites are subdivided into lineage compartments, each with distinct cell fates and evolutionary histories. Insights into somite evolution can come from studying amphioxus, the best extant approximation of the chordate ancestor. Amphioxus somites have myotome and non-myotome compartments, but development and fates of the latter are incompletely described. Further, while epithelial to mesenchymal transition (EMT) is important for most vertebrate somitic lineages, amphioxus somites generally have been thought to remain entirely epithelial. Here, we examined amphioxus somites and derivatives, as well as extracellular matrix of the axial support system, in a series of developmental stages by transmission electron microscopy (TEM) and in situ hybridization for collagen expression.

Results

The amphioxus somite differentiates medially into myotome, laterally into the external cell layer (a sub-dermal mesothelium), ventrally into a bud that forms mesothelia of the perivisceral coelom, and ventro-medially into the sclerotome. The sclerotome forms initially as a monolayered cell sheet that migrates between the myotome and the notochord and neural tube; subsequently, this cell sheet becomes double layered and encloses the sclerocoel. Other late developments include formation of the fin box mesothelia from lateral somites and the advent of isolated fibroblasts, likely somite derived, along the myosepta. Throughout development, all cells originating from the non-myotome regions of somites strongly express a fibrillar collagen gene, ColA, and thus likely contribute to extracellular matrix of the dermal and axial connective tissue system.

Conclusions

We provide a revised model for the development of amphioxus sclerotome and fin boxes and confirm previous reports of development of the myotome and lateral somite. In addition, while somite derivatives remain almost entirely epithelial, limited de-epithelialization likely converts some somitic cells into fibroblasts of the myosepta and dermis. Ultrastructure and collagen expression suggest that all non-myotome somite derivatives contribute to extracellular matrix of the dermal and axial support systems. Although amphioxus sclerotome lacks vertebrate-like EMT, it resembles that of vertebrates in position, movement to surround midline structures and into myosepta, and contribution to extracellular matrix of the axial support system. Thus, many aspects of the sclerotome developmental program evolved prior to the origin of the vertebrate mineralized skeleton.

Expression of carbonic anhydrase, cystic fibrosis transmembrane regulator (CFTR) and V-H(+)-ATPase in the lancelet Branchiostoma lanceolatum (Pallas, 1774)

Sequencing of the amphioxus genome revealed that it contains a basic set of chordate genes involved in development and cell signaling. Despite the availability of genomic data, up till now no studies have been addressed on the comprehension of the amphioxus osmoregulation. Using primers designed on Branchiostoma floridae carbonic anhydrase (CA) II, cystic fibrosis transmembrane regulator (CFTR) and V-H(+)-ATPase, a 100bp long region, containing the protein region recognized by the respective antibodies, has been amplified and sequenced in B. lanceolatum indicating the presence of hortologous V-ATPase, CFTR and carbonic anhydrase II genes in Branchiostoma lanceolatum. Immunohistochemical results showed that all three transporting proteins are expressed in almost 90% of epithelial cells of the skin in B. lanceolatum adults with a different degree of positivity in different regions of body wall and with a different localization in the cells. The comparison of results between young and adult lancelets showed that the distribution of these transporters is quite different. Indeed, in the young specimens the expression pattern of all tested molecules appears concentrated at the gut level, whereas in adult the gut loses its key role that is mostly supported by skin.

Origin and evolution of the integumentary skeleton in non-tetrapod vertebrates

Most non-tetrapod vertebrates develop mineralized extra-oral elements within the integument. Known collectively as the integumentary skeleton, these elements represent the structurally diverse skin-bound contribution to the dermal skeleton. In this review we begin by summarizing what is known about the histological diversity of the four main groups of integumentary skeletal tissues: hypermineralized (capping) tissues; dentine; plywood-like tissues; and bone. For most modern taxa, the integumentary skeleton has undergone widespread reduction and modification often rendering the homology and relationships of these elements confused and uncertain. Fundamentally, however, all integumentary skeletal elements are derived (alone or in combination) from only two types of cell condensations: odontogenic and osteogenic condensations. We review the origin and diversification of the integumentary skeleton in aquatic non-tetrapods (including stem gnathostomes), focusing on tissues derived from odontogenic (hypermineralized tissues, dentines and elasmodine) and osteogenic (bone tissues) cell condensations. The novelty of our new scenario of integumentary skeletal evolution resides in the demonstration that elasmodine, the main component of elasmoid scales, is odontogenic in origin. Based on available data we propose that elasmodine is a form of lamellar dentine. Given its widespread distribution in non-tetrapod lineages we further propose that elasmodine is a very ancient tissue in vertebrates and predict that it will be found in ancestral rhombic scales and cosmoid scales.

The musculotendinous system of an anguilliform swimmer: Muscles, myosepta, dermis, and their interconnections inAnguilla rostrata

Eel locomotion is considered typical of the anguilliform swimming mode of elongate fishes and has received substantial attention from various perspectives such as swimming kinematics, hydrodynamics, muscle physiology, and computational modeling. In contrast to the extensive knowledge of swimming mechanics, there is limited knowledge of the internal body morphology, including the body components that contribute to this function. In this study, we conduct a morphological analysis of the collagenous connective tissue system, i.e., the myosepta and skin, and of the red muscle fibers that sustain steady swimming, focusing on the interconnections between these systems, such as the muscle-tendon and myosepta-skin connections. Our aim is twofold: (1) to identify the morphological features that distinguish this anguilliform swimmer from subcarangiform and carangiform swimmers, and (2) to reveal possible pathways of muscular force transmission by the connective tissue in eels. To detect gradual morphological changes along the trunk we investigated anterior (0.4L), midbody (0.6L), and posterior body positions (0.75L) using microdissections, histology, and three-dimensional reconstructions. We find that eel myosepta have a mediolaterally oriented tendon in each the epaxial and hypaxial regions (epineural or epipleural tendon) and two longitudinally oriented tendons (myorhabdoid and lateral). The latter two are relatively short (4.5-5% of body length) and remain uniform along a rostrocaudal gradient. The skin and its connections were additionally analyzed using scanning electron microscopy (SEM). The stratum compactum of the dermis consists of approximately 30 layers of highly ordered collagen fibers of alternating caudodorsal and caudoventral direction, with fiber angles of 60.51 +/- 7.05 degrees (n = 30) and 57.58 +/- 6.92 degrees (n = 30), respectively. Myosepta insert into the collagenous dermis via fiber bundles that pass through the loose connective tissue of the stratum spongiosum of the dermis and either weave into the layers of the stratum compactum (weaving fiber bundles) or traverse the stratum compactum (transverse fiber bundles). These fiber bundles are evenly distributed along the insertion line of the myoseptum. Red muscles insert into lateral and myorhabdoid myoseptal tendons but not into the horizontal septum or dermis. Thus, red muscle forces might be distributed along these tendons but will only be delivered indirectly into the dermis and horizontal septum. The myosepta-dermis connections, however, appear to be too slack for efficient force transmission and collagenous connections between the myosepta and the horizontal septum are at obtuse angles, a morphology that appears inadequate for efficient force transmission. Though the main modes of undulatory locomotion (anguilliform, subcarangiform, and carangiform) have recently been shown to be very similar with respect to their midline kinematics, we are able to distinguish two morphological classes with respect to the shape and tendon architecture of myosepta. Eels are similar to subcarangiform swimmers (e.g., trout) but are substantially different from carangiform swimmers (e.g., mackerel). This information, in addition to data from kinematic and hydrodynamic studies of swimming, shows that features other than midline kinematics (e.g., wake patterns, muscle activation patterns, and morphology) might be better for describing the different swimming modes of fishes.

Architecture of the integument in lower teleostomes: functional morphology and evolutionary implications

A bony ganoid squamation is the plesiomorphic type in actinopterygians. During evolution, it was replaced by weak and more flexible elasmoid scales. We provide a comparative description of the integument of "ganoid" fishes and "nonganoid" fishes that considers all dermal components of mechanical significance (stratum compactum, morphology of ganoid scales, and their regional differences) in order to develop a functional understanding of the ganoid integument as a whole. Data were obtained for the extant "ganoid" fishes (Polypteridae and Lepisosteidae) and for closely related "lower" actinopterygians (Acipenser ruthenus, Amia calva) and "lower" sarcopterygians (Latimeria chalumnae, Neoceratodus forsteri). Body curvatures during steady undulatory locomotion, sharp turns, prey-strikes, and fast starts in "ganoid" fishes were measured from videotapes. Extreme body curvatures as measured in anesthetized specimens are never reached during steady swimming, but are sometimes closely approached in certain situations (sharp turns, prey-strike). During extreme body curvatures we measured high values of lateral strain on the convex and on the concave side of the body. Scale overlap changes considerably (66-127% in Lepisosteus, 42-140% in Polypterus). The ganoid squamation forms a protective coat, but at the same time it permits extreme body curvatures. This is reflected in characteristic morphological features of the ganoid scales, such as an anterior process, concave anterior margin, and peg-and-socket articulation. These characters are most pronounced in the anterior body region, where maximum changes in scale overlap are required. The anterior processes and anterior concave margin, together with the attached stratum compactum, guide movements in a horizontal plane during bending. Displacements of scales relative to each other are possible for scales of different scale rows, but are impeded in scales of the same scale row due to the peg-and-socket articulation. Furthermore, ganoid scale rows, fibers of collagen layers of the stratum compactum, and the lateral myoseptal structures follow the same oblique orientation, which is needed to achieve extreme body curvatures. There is no evidence that body curvatures are limited by the ganoid squamation in Polypterus or Lepisosteus to any larger extent than by a type of integument devoid of ganoid scales in teleostomes of similar body shape. Our results essentially contradict former functional interpretations: 1) Ganoid scales do not especially limit body curvature during steady undulatory locomotion; 2) They do not act as torsion-resisting devices, but may be able to damp torsion together with the stratum compactum and internal body pressure.

Sulphated polyanions in cytoplasm and nuclei of epithelial cells of Branchiostoma demonstrated by the cationic dye Cupromeronic Blue

The intracellular occurrence and distribution of sulphated polyanions, interpreted to represent mucins, were studied in secretory epithelial cells in the primitive chordates Branchiostoma lanceolatum and B. floridae at the electron microscopical level by using Cupromeronic Blue (CMB). CMB-precipitates were mainly found within two potential types of mucin vesicles (apical and basal) and Golgi cisterns. The mucin vesicles form a distinct population of secretory granules different from another nonmucin granule population. Within the epidermal cells the staining intensity of the Golgi cisterns with CMB increased from the cis to the trans compartment. The pharyngeal mucous cells showed staining only in the trans Golgi compartment. These findings indicate, that CMB can be used for intracellular localization of mucins and that sulphation of the mucins in the investigated cells may occur within different compartments of the Golgi complex. Apparently the mucin is secreted apically but only in the epidermis it forms a dense layer covering the apical microvilli. In the Branchiostoma epidermal cells a layer of specialized basal vesicles occurred, containing unusually large and branched CMB-precipitates which possibly serve mechanical functions. In the nuclei CMB-precipitates were regularly demonstrated in the euchromatin of the cell types studied.

Intermediate filaments in non-neuronal cells of invertebrates: isolation and biochemical characterization of intermediate filaments from the esophageal epithelium of the mollusc Helix pomatia

To screen invertebrate tissues for the possible expression of intermediate filaments (IFs), immunofluorescence microscopy with the monoclonal antibody anti-IFA known to detect all mammalian IF proteins was used (Pruss, R. M., R. Mirsky, M. C. Raff, R. Thorpe, A. J. Dowding, and B. H. Anderton. 1981. Cell, 27:419-428). In a limited survey, the lower chordate Branchiostoma as well as the invertebrates Arenicola, Lumbricus, Ascaris, and Helix pomatia revealed a positive reaction primarily on epithelia and on nerves, whereas certain other invertebrates appeared negative. To assess the nature of the positive reaction, Helix pomatia was used since a variety of epithelia was strongly stained by anti-IFA. Fixation-extraction procedures were developed that preserve in electron micrographs of esophagus impressive arrays of IFs as tonofilament bundles. Fractionation procedures performed on single cell preparations document large meshworks of long and curvilinear IF by negative stain. These structures can be purified. One- and two-dimensional gels show three components, all of which are recognized by anti-IFA in immunoblotting: 66 kD/pl 6.35, 53 kD/pl 6.05, and 52 kD/pl 5.95. The molar ratio between the larger and more basic polypeptide and the sum of the two more acidic forms is close to 1. After solubilization in 8.5 M urea, in vitro filament reconstitution is induced when urea is removed by dialysis against 2-50 mM Tris buffer at pH 7.8. The reconstituted filaments contain all three polypeptides. The results establish firmly the existence of invertebrate IFs outside neurones and demonstrate that the esophagus of Helix pomatia displays IFs which in line with the epithelial morphology of the tissue could be related to keratin IF of vertebrates.

Septate junctions with paired septa in the cephalochordate Branchiostoma lanceolatum

The cells of the atrial epithelium of Branchiostoma lanceolatum are interconnected by an apical zonula adhaerens and a septate junction extending between the apical zonula adhaerens and a level corresponding to the middle of the nucleus. The spacing of the septa, which are relatively few in number (about 10), varies considerably. Within the junction paired and unpaired septa occur. The thickness of the paired septa measures 16-25 nm, the distance between the individual septa of the paired structure 6-12 nm, and the intercellular space at the site traversed by the septa 17-20 nm. At the intersection between three cells the septa (paired or unpaired) delineate a central triangular space.